In magnetic resonance (MR) imaging systems, a main magnet is used to generate a strong static magnetic field. In order to perform MR measurements with a good accuracy, it is required that the static magnetic field is homogenous in a volume of interest. The volume of interest corresponds to an examination space of the MR imaging system and is typically a spherical or ellipsoid space with a diameter of about 50 centimeters. A variation of the static magnetic field within the volume of interest of less than 20 ppm is generally required. Before field homogeneity correction (otherwise known as shimming), a typical main magnet can have an inhomogeneity of about 500 ppm. Adjustment of the magnetic field is required, e.g. by adding magnetic material within the main magnet or by setting appropriate currents in adjustment coils. Before such corrections can be made, an accurate measurement of the magnetic field inside the magnet is needed.
Determining the static magnetic field inside the volume of interest which is usually a sphere or spheroid space located in the centre of the magnet, is also referred to as field mapping. This field mapping of an MR imaging system involves accurate determination of the magnetic field in a large number of locations. In known methods for determining the static magnetic field, the field is sampled over a closed surface bounding the volume of interest; if the field on the surface of the volume of interest is known, it can be reconstructed inside the entire volume enclosed by this surface. Measurements of the magnetic field are performed in 12-24 concentric circles about the longitudinal axis of the main magnet, also referred to as z-axis. Each circle is provided in a plane rectangular to the z-axis, and the measurements are provided in an angular distance of 15-30 degrees around the z-axis.
Conventional measurement methods employ a Nuclear Magnetic Resonance (NMR) magnetometer or an array of such magnetometers. The NMR magnetometer is moved within the static magnetic field of the main magnet to desired sample locations in order to perform the required measurements as specified above. The movement is realized by use of a holding apparatus, which is usually mechanically operated in order to reduce influences on the static magnetic field.
The NMR magnetometer and the holding apparatus to move the NMR magnetometer(s) are complicated to handle and expensive. The method for mapping the MR magnetic field using the NMR magnetometer and the holding apparatus is time consuming and also difficult to execute. Accordingly, an improvement is desired.
An additional problem arises since the static magnetic field is influenced by the location where the MR imaging (MM) system containing the main magnet is located. Therefore, the homogeneity of the main magnet has to be verified every time the MRI system is moved, in particular when the system is installed at a new location. Accordingly, availability of the NMR magnetometer and the holding apparatus are reduced by way of transport time, and there is an increased risk of damaging or loss of the NMR magnetometer and the holding apparatus during transport. The same problem arises each time maintenance of the main magnet has to be performed, i.e. each time the static magnetic field is calibrated.